Diamond-like coatings are in great demand as protective coatings for optical components; as coatings in sliding wear parts such as valves, pistons, bearings and the like; as heat sinking materials in integrated circuit technology and as laser host material. Diamond-like coatings also have great potential as a integrated circuit semi-conductor material because of the very high heat transfer of such materials. By diamond-like coatings, applicant means coatings formed of carbonaceous species having characteristics of hardness and chemical structure similar to natural diamond, at least in part. Such coatings may include other species of chemicals and structure, at least in part.
Because of the great demand for these materials, techniques have been developed to produce this material. Common to all of the production techniques is the formation of a gas plasma as a source of free radicals and ions which form the depositing species and/or coating environment and which provides the transient energy required for diamond or diamond-like film nucleation. The conventional methods require radio frequency electronics to generate the plasma and require suitable filtering and collimating electrodes to extract and control the depositing species.
In addition, conventional methods require the provision of a means for insuring an electrical potential difference between the plasma region and the substrate to be coated, normally in the range of 100 to 1,000 electron volts potential difference. A further problem common to conventional methods of formation of diamond-like coatings is the requirement that adequate cooling be provided for the substrate, since the substrate is in close proximity to a very hot plasma. Conventional processes require that the substrate be maintained near room temperature as a key ingredient to forming diamond-like coatings. Heating of the coating films, either during formation or afterwards, will collapse the sp3 bonding characteristic of diamond films to a sp2 bonding, that is, will convert the diamond-like film to one of graphite.
Diamond-like coatings formed by state of the art methods notoriously have problems with adhesion to the substrate and are unable to support a shear stress. The industry recognizes a ten micron thickness limit as that which is attainable by conventional processes. The accumulation of diamond-like material above this limit incurs problems with the cohesive forces within the film which induces delamination of the film. Moreover, attempts to produce film beyond the ten micron thickness result in the collapsing of the sp3 hybrid to the sp2 bonding, that is, the formation of graphite. These films, above the ten micron thickness, have a very weak bond to the substrate such that the diamond-like coating can be easily scraped from the substrate, for example by application of a dental pick.
Conventional plasma assisted techniques for producing diamond-like coatings grow coatings which have separate, granular crystal structure, that is, an irregular geode-like appearance on microscopic examination. These films of discrete crystaline nature are unsatisfactory particularily when used as optical coatings since scattering of light as a result of the separate crystals can degrade the optical throughput.
With this process, continuous, tightly adhering, optically suitable diamond-like films may be deposited on substrates such as lenses, or on other surfaces subject to wear, without the use of the electronics and necessary in state of the art deposition processes. This method generates a plasma by the absorbtion of laser radiation into a precurser gas or gas mixture. The gas rapidly decomposes to produce the accelerating forces and the depositing species necessary to produce diamond-like coatings. Externally generated accelerating fields or potentials between the gas and the substrate are not required. Moreover, it is not necessary to substantially heat the substrate prior to deposition. Instead, the plasma region may be spaced from the substrate and generated intermittently, for example at ten microseconds duration, so that the temperature excursions are transient and high energies are developed to propel the deposition species onto the substrate. This system does not require a sophisticated vacuum system, as do conventional methods, since the coatings may be produced at atmospheric pressure as well as at reduced pressured. For example, applicant's process may typically operate at one to 100 torr total pressure. Conventional deposition processes normally take place at millitorr pressures.
This process may also work to provide deposition coverage of substantially greater areas than previous techniques. Previously, the art required collimated beams or very localized plasma mixtures which result in small area coverage. This process may be used over much wider areas. Moreover these films may be much thicker than conventional films. This process has produced films of greater than ten microns thickness. At this thickness the films have very high adhesion to the substrate. Standard pull tests indicate that the films may withstand in excess of 10,000 psi pulling pressure. The films are also very strong in shear stress. A Sebastian pulling post can be pulled through twenty degrees on removal of this film.
In addition to coating flat substrates, this process may produce coatings over curved or irregular surfaces as well. Coated surfaces of this type would be highly valuable on curved lenses, on nosecones, aircraft canopies and on windows. The films have a high degree of lubricity as well as hardness and durability.
It is an object of this invention to produce tightly adhering diamond-like coatings on substrates and to produce substrates having the diamond-like coatings thereon.
It is a further object of this invention to produce diamond-like coatings having superior optical, electrical, electronic, thermal and mechanical properties.
It is also an object of this invention to produce diamond-like coatings from a laser induced plasma and to develop a method for producing such coatings.
It is an object of this invention to develop a method of producing diamond-like coatings by absorbing laser produced energy into a coating precurser gas.
It is an object of this invention to develop a method of producing diamond-like coatings by absorbing laser energy into a plasma initiator.
It is an object of this invention to develop a method of producing diamond-like coatings from a precursor gas by initiating and propagating a laser induced plasma therein.
These and other objects will be apparent from the following Description of the Drawings and Description of the Preferred Embodiments.